1. Field of the Invention
The present invention relates to a display device for projecting images on a screen using light valve elements such as liquid crystal panels or reflective liquid crystal elements and relates in particular for example, to an image display device for liquid crystal projector devices, reflective image display projector devices, liquid crystal televisions and projection type display devices, etc.
2. Description of the Related Art
Projection type display devices such as liquid crystal projectors are known in the related art as a means to irradiate light from a light source such as an incandescent bulb onto light valve elements such as a liquid crystal display panel for projecting an enlarged image.
In image display devices of this type, light from a light source is changed and adjusted for brightness and darkness on each pixel in the light valve element and projected on a screen, etc. In twisted nematic (TN) type liquid crystal display devices constituting typical liquid crystal display elements, two polarizing plates are each installed at mutually different 90 degree light polarization directions, in the front and rear of the liquid crystal cell formed by injecting liquid crystals between a pair of transparent substrates having a transparent electrode film, and by combining the effect from selecting polarized light constituents of the polarizing plate and rotation of the deflection plane by the electro-optical effect of the liquid crystal, the permeable light intensity of the input light is controlled and image information is displayed. In recent years, rapid progress has been made in making these permeable or reflective image display elements themselves more compact and improving performance such as resolution.
The advancements in making display devices using light valve elements such as image display elements more compact and having high performance had led not just simply to making image displays with video signals as in the related art, but proposal of technology for projector type image display devices constituted by image output device for personal computers. Demands here stress compactness and obtaining a bright image extending to all corners of the screen. However, projector type image display devices of the related art have the drawbacks of being large and that the image brightness ultimately obtained and performance characteristics such as image quality are inadequate.
For example when making the overall liquid crystal display device more compact, an effective method is to make the light valve elements or in other words, the liquid crystal display elements themselves smaller. However, when the liquid crystal display elements are made smaller, the surface area irradiated by the liquid crystal means becomes smaller. Consequently, the surface area struck by the lighting means for the total luminous flux intensity emitted by the light source become smaller so the problem occurs that the percentage of luminous flux intensity (hereafter light utilization efficiency) on the liquid crystal element was low to the total luminous flux intensity emitted by the light source. Another problem is that the sides of the screen are dark. Further, the liquid crystal display element can only utilize the polarized light in one direction so that only approximately half of the random polarized light emitted from the light source is utilized. Technology for an optical system to beam random polarized light from a light source on a liquid crystal display element aligned in a one-way polarization direction, is disclosed in Japanese Patent Laid-Open No. H4-63318 wherein a polarity converter element such as a polarized beam splitter is utilized and random polarized light beamed from a light source is separated into P polarized light and S polarized light and combined together using a prism.
The optical system of the related art utilizing the above arrangement, and particularly a lighting system utilizing a reflective liquid crystal display device was configured so that the polarized beam splitter and reflective liquid crystal display element were combined and the light polarization direction converted and checked according to the expressed tones and the on/off of the video, and the video later projected onto a screen by a projecting lens.
Due to the polarized beam splitter, the above configuration had the problems that irregularities occurred in the color and the contrast was low.
In other words, changes occurred in the permeance rate of the P polarized light to the angle of the input light beam and the reflection rate of the S polarized light so that irregularities occurred in the reflection rate and permeance rate of the polarized beam splitter to the specified angle of the lighting system. These irregularities caused deterioration in the quality of the image quality projected on the screen.
The polarized beam splitter such as disclosed in Japanese Patent Laid-Open No. 09-054213 with the permeant material enclosing the PB film was comprised of glass material with an optical resilience coefficient having an absolute value within 1.5×10−8 cm2/N, so that the birefringence (double refraction) was low and the contrast on the screen was improved.
However, in this example of the related art, the weight of the polarized beam splitter glass material itself was heavy (more than twice the conventional weight), the utilization level was preferably low since the cost was high. However, in typical optical systems other than the embodiment of this invention, three R G B reflective panels were used and each required a polarized beam splitter so that no consideration was given to reducing the size, the weight or the cost of the optical system.
Also, in optical systems utilizing reflective liquid crystal display elements, the dichroic mirrors or dichroic prisms made with a dichroic coating and utilized for color separation or combination, changed the direction of the light by means of polarizing the direction of the light when beaming light in a system for color separation and combination. The characteristics are known to change due to the polarization direction of light beamed onto the dichroic coating. In other words, a difference in light wavelength bands occurs in light separated into P polarized light and S polarized light. More specifically, on a dichroic blue reflective surface, the half wavelength of a P polarized light input beam is lower than an S polarized light input beam. In such a case, the beam input with S polarized light is separated into permeable light and reflected light according to the S polarized light half wavelength λ s by the blue reflective coating surface. When the image information is white, the light is converted into P polarized light by the blue reflective liquid crystal display element, and the light beam input again onto the blue reflective coating surface. This time the beam input with P polarized light is separated into permeable light and reflected light according to the polarized light half wavelength λ p. In this case, the half-wavelength portion that has fallen low is not reflected back and is a permeable part of the wavelength band. The light on the permeating part of the wavelength band cannot be utilized in the image display device so the light half-wavelength differential is lost and the brightness diminishes and color performance deteriorates. The same effects occur on the red reflective surface.
Therefore the light that deviates from this wavelength band cannot be utilized. The problem of lowering of the light utilization efficiency and a deteriorated color performance therefore occur in the image display device.
Contrast is an important performance characteristics in image display devices, and inserting a polarizing plate between both or either of the polarized beam splitter and lighting system, and polarized beam splitter and projection lens is effective in improving contrast. However, in the related art, all the red, blue and green light permeates through the polarizing plate creating the problem of a rise in temperature in the polarizing plate, a drop in contrast, and burns on the polarizing plate.
Therefore, as can be seen from the above description, measures must be taken to reduce the size of the optical system and projection image display system itself as well as reduce weight and reduce costs while maintaining the image quality and the brightness of the image display device.